The present invention generally relates to compound semiconductor crystal-on-substrate structures and methods of producing the same, and more particularly to a compound semiconductor crystal-on-substrate structure having a group III-V compound semiconductor layer such as a GaAs layer formed on a Si substrate and to a method of producing such a compound semiconductor crystal-on-substrate structure.
Semiconductor devices using a group III-V compound semiconductor crystal layer such as a GaAs crystal layer can operate at a high speed compared to semiconductor devices using Si, because of the large carrier mobility. In addition, the bandgap enables emission of visible light, thereby making the semiconductor devices using the group III-V compound semiconductor crystal layer extremely useful. However, it is difficult to produce a substrate having a large diameter for the semiconductor devices using the group III-V compound semiconductor crystal layer, and the mechanical strength of the substrate is poor. For this reason, the development of the semiconductor devices using the group III-V compound semiconductor crystal layer has been slow compared to the development of the semiconductor devices using Si.
Accordingly, proposals have been made to grow a GaAs crystal layer on a Si substrate which has a satisfactory mechanical strength and can be made to a relatively large diameter. According to such proposals, a light emitting diode (LED), a laser, a field effect transistor (FET) or the like is formed on the GaAs crystal layer.
However, considerable crystal defects existed in the group III-V compound semiconductor crystal layer such as the GaAs crystal layer that is formed on the Si substrate according to the above proposals. For this reason, it was impossible to produce the LED, laser, FET or the like having satisfactory characteristics using the group III-V compound semiconductor crystal layer.
In order to reduce the crystal defects in the group III-V compound semiconductor crystal layer such as the GaAs crystal layer formed on the Si substrate, it was conceivable to first form the group III-V compound semiconductor crystal layer to a desired thickness on the Si substrate, and then carry out a cyclic thermal process. Alternatively, it was conceivable to first form the GaAs crystal layer to approximately one-half the desired thickness on the Si substrate, then carry out a cyclic annealing process, and continue growing the GaAs crystal layer to the desired thickness.
However, according to such conceivable methods, it was difficult to reduce the crystal defect density to 1.times.10.sup.6 cm.sup.-2 or less.